1. Field of the Invention
The present invention relates to a television camera apparatus which has a solid-state imaging device such as a CCD (Charge Coupled Device), and more particularly to a technique of improving vertical resolution.
2. Description of the Related Art
A brief discussion of interlace scanning will now be entertained. 2:1 interlace scanning is employed in the color television broadcasting of a NTSC system or a PAL system. Television cameras having a CCD (hereinafter called "CCD cameras") used in color television broadcasting adopt a so-called "reading of signals of two-pixel-summation" for both a first field and a second field. In reading signals using two-pixel-summation, signals are read from about 480 scanning lines (in the NTSC system) or from about 580 scanning lines (in the PAL system) in the available picture zone.
Furthermore, in reading signals of two-pixel-summation, the charges of two adjacent pixels in any row, which are incorporated in the CCD, are added together to form a signal. This signal, which is output from the CCD, is equivalent to the sum of two video signals generated from two adjacent scanning lines as in a so-called "reading of signals of two-line-summation."In a CCD camera, the combination of two pixels accumulating charges to be added is changed for each field to alter combination of two scanning lines. Thus interlace scanning is performed.
In reading signals of two-line-summation, an output of the CCD is a sum of the outputs generated from two adjacent scanning lines. Hence, this scheme enhances the sensitivity of the CCD camera. The reading of signals of two-line-summation, however, is disadvantageous in that each scanning line becomes equivalently thick. In addition, since two lines are read as one, the resulting scanning lines appear to be fewer and farther apart. As a result, the vertical resolution of the CCD camera is decreased.
How each scanning lines becomes equivalently thick will now be explained, with reference to FIGS. 1, 2, and 3.
FIG. 1 shows an image which is an object of photography for the CCD camera. The image consists of a white square on a rectangular black background. It is assumed here that the background is not exactly black and therefore has some reflectivity. In FIG. 1, the solid lines indicate scanning lines for odd-numbered fields (hereinafter referred to as "first fields"), and the broken lines indicate scanning line for even-numbered fields (hereinafter referred to as "second fields"). (Generally, a solid line and a broken line are used to indicate a scanning for an odd-numbered field and an even-numbered field, respectively to explain interlace sanning.) The vertical dot-dash line X--X' traversing near the center of the image indicates a sampling position where the vertical resolution of the CCD camera is measured.
FIG. 2 shows the levels of video signals sampled at the intersections (i.e., pixels) of the line X--X' and the scanning lines. More precisely, the levels of the signals are plotted on the ordinate and along the abscissa which is a time axis. Each vertical solid line represents a video signal for the first field, whereas each vertical broken line indicates a video signal for the second field. The letters "a" and "b" denote an odd-numbered scanning line and an even-numbered scanning line, respectively. The envelop passing through the peaks of the sampled video signals represents the distribution of signal levels along the line.
When the CCD camera reading signals of two-line-summation scans the image shown in FIG. 1, the video signals sampled at the intersections of the line X--X' and the solid lines will define the first-field level distribution shown in FIG. 3A. Similarly, the video signals sampled at the intersections of the dot-dash line and the broken lines will define the second-field level distribution shown in FIG. 3B. All video signals sampled at the intersections of the line X--X' and the solid and broken lines will define the overall level distribution shown in FIG. 3C.
For the first fields, video signals b.sub.1, b.sub.2, b.sub.3, . . . shown in FIG. 2 are added to video signals a.sub.1, a.sub.2, a.sub.3, . . . shown in FIG. 2, respectively, thereby forming the video signals a'.sub.1, a'.sub.2, a'.sub.3, . . . . which are illustrated in FIG. 3A. For the second fields, video signals a.sub.1, a.sub.2, a.sub.3 . . . . shown in FIG. 2 are added to video signals b.sub.1, b.sub.2, b.sub.3 . . . . shown in FIG. 2, respectively, thereby forming the video signals b'.sub.1, b'.sub.2, b'.sub.3, . . . . which are illustrated in FIG. 3B. The signals a'.sub.1, a'.sub.2, a'.sub.3, . . . . and the signals b'.sub.1, b'.sub.2, b'.sub.3, . . . . are output from the CCD camera.
When the output of the CCD camera is interlaced, the first-field level distribution and the second-field level distribution become interpolated as shown in FIG. 3C. The CCD camera output, thus processed, is supplied to a television receiver, and the CRT of the receiver displays a horizontal line in the level distribution defined by the envelope shown in FIG. 3C.
The leading edge of the envelope shown in FIG. 2 extends from the sampling points b.sub.2 to b.sub.4, whereas that of the envelope shown in FIG. 3C extends for a longer time, from the sampling points b'.sub.2 to b'.sub.4. This is because the reading of signals of two-line-summation is employed in the CCD camera. For the same reason, the trailing edge of the envelope of FIG. 3C extends for a longer time than that of the envelope of FIG. 2. The increase in the time over the leading and trailing edges of the envelope means a decrease in vertical resolution. Needless to say, this decrease in vertical resolution is the result from the application of reading of signals of two-line-summation.
In order to prevent such a decrease in vertical resolution, the CCD camera may adopt a technique of reading signals of one line in which video signals a.sub.n and b.sub.n are read in the first field and the second field, respectively. In this case, however, the charges in any two adjacent pixels are not added, and the sensitivity of the CCD camera is consequently reduced to 50%.
Jpn. Pat. Appln. KOKAI Publication No. 2-220574 (Title: "CCD Camera Apparatus") discloses a method of preventing a decrease in vertical resolution.
In the Kokai method, video signals generated from two adjacent horizontal lines are read from a CCD simultaneously but independently. In each odd-numbered field, the video signal produced from the odd-numbered line is used as a basic video signal, whereas the video signal generated from the even-numbered line is used as a first comparative video signal. The first comparative video signal is delayed for one horizontal-scanning period, producing a second comparative video signal. The second comparative video signal is compared with the next basic video signal. Similarly, in each even-numbered field, the video signal produced from the even-numbered line is used as a basic video signal, whereas the video signal generated from the odd-numbered line is used as a first comparative video signal. The first comparative video signal in the even-numbered field is also delayed by one horizontal-scanning period, producing a second comparative video signal, which is compared with the next basic video signal. In each field, both odd-numbered and even-numbered, the difference between the basic video signal and the first comparative video signal and the difference between the basic video signal and the second comparative video signal are obtained, forming two difference signals. The difference between the difference signals is then obtained and added as a vertical-edge signal to the basic video signal in each field to emphasize a vertical edge.
This method is basically that of the reading of signals of one line. Hence, the CCD camera has only about 50% of the sensitivity it can attain in adopting the reading of signals of two-line-summation.
In another technique, the CCD camera has the capability to process signals without having its sensitivity reduced, even if operated in reading signals of one line.
The charges in the pixels are read during field periods in the reading of signals of two-line-summation. In an NTSC system, the charges are read every 1/60 sec if the field periods are 1/60 sec. When this is the case, the charge-intergration period of the CCD is 1/60 sec. In reading signals of one line, it suffices to read a scanning line for odd-numbered fields (indicated by a solid line in FIG. 2) in the first field. No scanning line needs to be read in the second field which follows the first. Hence, the charge-intergration period of the CCD can be lengthened to 1/30 sec. (The period thus lengthened is generally known as "frame-integration period.")
Doubling of the charge-intergration period, however, gives rise to an afterimage when used for motion pictures. In fact, when the charge-integration period increases to the frame-integration period, afterimage occurs and the quality of moving pictures deteriorates.
A method of improving the vertical resolution of a CCD camera, without causing afterimage is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 63-209280 (Title: "Method of Driving the Electronic Shutter of a Solid-State Imaging Apparatus"). In this method, the shutter of the CCD camera is driven such that the period for which two adjacent charge-integration elements located on an odd-numbered line accumulates charges and the period for which two adjacent charge-integration elements located on an even-numbered line accumulates charges are altered in every field. The vertical resolution of the CCD camera is thereby enhanced.
This method is, however, disadvantageous in the following respects since the CCD shutter is driven in a specific manner to improve the vertical resolution.
(1) The more the vertical resolution is improved, the lower the level of the signal output from the CCD. PA0 (2) Ordinary shutter driving cannot be performed. PA0 (3) Light and dark stripes cannot be prevented from appearing in the image which the CCD camera provides (i.e, a re-take image) by taking a TV-screen image displayed by a TV receiver synchronous with the camera. (It is known that no stripes will appear in a re-take image if the CCD shutter is not driven, whereas stripes will appear in the image if the CCD shutter is driven.) PA0 (4) An image of an object illuminated with light from a 50 Hz fluorescent lamp light and photo graphed by the CCD camera cannot be free of flicker. This is because the shutter cannot be driven at the frequency of 100 Hz to eliminate the effect of the fluorescent-lamp light on the image.
A decrease in the level of the CCD output signal, i.e., the problem (1), accompanies the ordinary use of the CCD camera. A decrease in the signal level may reduce the sensitivity of the CCD camera, which is inversely proportional to the resolution.
The above-described method of improving the vertical resolution of a CCD camera can indeed shorten the time over the leading and trailing edges of the envelope shown in FIG. 3C. This method, however, cannot add a vertical-edge signal to a basic video signal.
In conventional cameras employing interlace-scanning, vertical-edge signals are generated and are used to achieve image-edge correction. Here resides a problem. The scanning line corresponding to each vertical-edge signal is substantially thick since the signal has been produced from three signals for the same field, i.e., a signal not delayed at all, a signal delayed by a horizontal-scanning period, and a signal delayed by two horizontal-scanning periods.
Jpn. Pat. Appln. KOKAI Publication No. 2-31912 (Title: "Apparatus for Generating Vertical-Edge Signals") discloses a method in which a field memory is used to process signals for producing a vertical-edge signal. The scanning line corresponding to this vertical-edge signals has a thickness which is half the thickness of the scanning line corresponding to a signal produced from those three signals described above. This method requires not only a field memory but also digital processing circuits. Furthermore, with this method it is necessary to obtain a difference between two signals delayed by a one-field period and another delay by a two-field period. It is also necessary to process afterimage signals generated from the over lapping portions of the images which are represented by those two signals.